The present invention relates to acousto-optic devices, and more particularly to a method and apparatus for increasing the angular aperture of an acousto-optic device. An important characteristic of any acousto-optic device is the angle of incidence through which light may be applied to the acousto-optic device without degrading the resolution of the acousto-optic device. This angle of incidence is known as the angular aperture of an acousto-optic device. A large angular aperture or acceptance angle is desirable since this results in an increased light gathering capability of the acousto-optic device.
The maximum aperture of an acousto-optic device is determined by the allowable phase mismatch between the incident optical light and the acoustic waves, beyond which the diffraction efficiency of the acousto-optic device drops to under one-half the value for the exact phase matching (i.e., exact Bragg angle matching).
The design parameters of an acousto-optic device such as an acousto-optic tunable filter (hereinafter "AOTF") include the angle of incident light with respect to the crystal axis of the material comprising acousto-optic device, .theta..sub.i, and the interaction length of the incident light and acoustic waves travelling within the crystal, L. Generally, the interaction length L is approximately the same as the length of a transducer launching acoustic waves into the crystal.
FIG. 1 illustrates the relationships of these parameters. In FIG. 1, reference symbol .DELTA..theta..sub.i denotes the angular aperture of the angular aperture .DELTA..theta..sub.i to L is ##EQU1## wherein .lambda..sub.o is the wavelength of light travelling within the AOTF, and .DELTA.n is the birefringence of the crystal material comprising the AOTF. For example, if the crystal material comprises thallium arsenic selenide (Tl.sub.3 AsSe.sub.3) (hereinafter "TAS"), the birefringence is about 0.18. From equation 1, it is seen that the angular aperture can be made large by making L small. However, when L is small, a high RF drive power is required to operate the acousto-optic device so as to achieve an acceptable diffraction efficiency. This is because both the diffraction efficiency of an acousto-optic device and the drive power are related to the length L of the acoustic transducer. Diffraction efficiency is a well known quantity and is discussed in I. C. CHANG, "Acousto-Optic Devices and Applications, " IEEE Trans. on Sonics and Ultrasonics, Vol. SU-23 No. 1, pp. 2-21, January 1976, and in Gottlieb et al., Electro Optic and Acoustic Optic Scanning and Deflection, Marcel, Dekker, 1985, at, for example, page 110, Equations 6.24 and 6.25.
Generally, for a given drive power density, as the length of the acoustic transducer L increases, the diffraction efficiency improves. Therefore, it is undesirable to make L small, because the power drive requirements therefor are great. In short, the smaller the length L, the greater the needed power density. As a result, with small transducer lengths the transducer tends to overheat. For example, if 5 watts are needed for an acousto-optic device, applying this power to a large transducer provides a low power density. But, when applying it to a small transducer the power density may be too high for the transducer. Therefore, making L small limits the amount of power that can be applied to the transducer. As a result, the angular aperture of an acousto-optic device cannot be greatly improved by making the length of the transducer L too small to support the required power.